Luke Milliron, UCCE Farm Advisor Butte, Glenn, and Tehama Counties
Janine Hasey, UCCE Farm Advisor Emerita
In the Sacramento Valley, an autumn freeze has damaged walnut orchards every November for the last three years. Although sudden autumn freezes are nothing new for walnut growers, it is potentially unprecedented to have severe autumn freezes three years in a row! Now, preparing for these freeze events needs to be a regular part of every walnut grower’s summer and fall orchard operations.
How to reduce future damage.
- For mature trees, don’t apply any additional nitrogen (N) from September onwards to prevent tender new growth that is most vulnerable to freeze damage. For young trees, it’s best to cut off N applications by mid-August.
- For young trees, withhold irrigation starting in early to mid-September, waiting to resume irrigation until a terminal bud (photo 1) is set on the trunk. After the terminal bud has set, irrigation can resume to avoid tree stress and defoliation, without the fear of pushing tender new growth.
For bearing trees, terminal buds may set as a positive side effect of the water cutoff ahead of harvest done to avoid shaker injury.
- If there has not been adequate rainfall by the end of October, irrigate both young and mature orchards so the soil is moist going into November. Trees with adequate soil moisture are better able to withstand low temperatures without damage than trees in dry soil because water-filled spaces in the soil conduct and store more heat than empty airspaces. To know if rainfall is meeting this moisture need or if irrigation is called for, compare rainfall totals with ET, and monitor soil moisture levels by hand or with sensors.
- Continue to actively monitor soil moisture and freeze predictions in November and December. If a freeze is predicted and the soil is dry, it should be wetted 2 to 3 days prior to a freeze event to fill the air spaces so the soil will store more heat. The top foot of soil is the most important and should be at field capacity (not saturated/ponding). Either a dry surface crust on one extreme or a frozen sheet of ponded water on the other extreme will both hinder the re-radiation of stored daytime heat during the night.
- If you suspect freeze damage occurred, cut into the branches in the fall or winter and check the tissue for drying or browning (photo 2). Swift action in the week after the freeze event can significantly decrease damage. Sunburn after freeze can further damage tissue on the southwest side of the tree. Paint the southwest side of damaged trees with 50% diluted (1:1 water to paint) white interior latex paint. Painting up to a week after the freeze event can reduce additional damage by half or more.
The November 2018 freeze caught several walnut growers with large and small acreages in the Sacramento Valley by surprise when severe freeze damage was seen in spring 2019. After these growers adapted their management practices, particularly with postharvest irrigations in early November when there was no rainfall, their trees survived the freeze events in 2019 and 2020 largely unscathed. In 2020-21 because of severe drought conditions, a Butte County walnut grower irrigated regularly throughout fall and winter to maintain constant soil moisture and attributed this practice to his success of avoiding severe freeze damage.
Drought is a clear and direct challenge to preparing for the next sudden autumn freeze. Let’s hope for early November rainfall since it is likely that growers with only surface water sources may not be able to irrigate this November.
We encourage you to participate in this survey provided by University of California Cooperative Extension (UCCE) regarding freeze damage in walnuts. The purpose of this survey is to gain greater understanding of freeze damage in walnuts.